This application claims the priority of German patent document 199 24 862.1, filed May 31, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for setting a specifiable target speed for a vehicle.
Speed-regulating systems are known, in which a target speed specified by the driver or calculated by means of an automatic determination system can be maintained automatically in a motor vehicle. These speed-regulating systems include, for example, cruise-control functions, in which the intention is that the vehicle should maintain a minimum speed selected by the driver, and limiter functions, where the vehicle must not exceed a maximum speed specified by the driver or determined automatically in spacing control systems. To implement the limiter function, actuating signals are produced in the speed-regulating system on the basis of the maximum speed and the current vehicle state variables, in particular the current vehicle speed, and these signals are used to adjust the engine, the gearbox and/or the wheel brakes to maintain the target speed.
In the speed-regulating system, the deviation of the actual speed from the specified desired value is fed to a controllerxe2x80x94generally a PDT1 , controllerxe2x80x94in which an actuating signal is produced to adjust a throttle valve in the intake duct of the internal combustion engine. To achieve linear conversion of the controller output into an associated engine torque, the controller output is first passed via a metering characteristic, which compensates for nonlinearities in the engine torque curve, and then fed as an actuating signal to an actuator of the throttle valve.
The metering characteristic and the parameters of the controller used are configured for optimum controller behaviour.
German patent document DE 44 39 424 C1 discloses a driving-speed controller which produces actuating signals that are manipulated further at the controller output in order to improve controller behaviour, particularly in low engine-load ranges. At the output of the controller there is an attenuation element which attenuates the controller signal in the lower value range before it is passed to a fuel-quantity setting element, thereby suppressing unwanted engine-speed oscillations at low engine load and improving driving comfort. Further possibilities for use and control are not disclosed by this publication, however.
On the other hand, it is also known to shut down one of two cylinder banks of a multi-cylinder internal-combustion piston engine in a partial load operation, while performing combustion in both cylinder banks in full-load operation. An internal combustion engine of this kind is disclosed, for example, in German patent document DE 196 11 363 C1. Here, there is the problem that the internal combustion engine has different transmission behavior in partial load operation and in full-load operation, and this can lead to controller instabilities when switching over between partial and full cylinder ignition in regulated speed operation. The controller parameters and the metering characteristic therefore must be adapted to match each type of operation.
German patent document DE 33 34 720 C2 describes a control apparatus in which cylinder shut-down is carried out only at steady-state operating points of the engine. Although this technique reduces the risk of instabilities, it also limits the range of application to these operating points. Thus, the advantage of cylinder shut-down (reduced fuel consumption) can also be exploited only to a limited extent.
One object of the present invention is to achieve improved controller behaviour in speed-regulating systems for vehicles with internal combustion engines which are equipped with partial cylinder shut-down.
This and other objects and advantages are achieved by the method according to the invention, in which both full operation of the internal combustion engine and partial cylinder shut-down operation are provided with respective metering characteristics, which provide an association between the controller output and the throttle-valve position in both types of operation of the engine.
In such a system, a jump in the actuating signal acting on the throttle valve in the case of a switchover when changing between full operation and partial shut-down, would lead to an abrupt change in the position of the throttle valve and to an undesirable torque jump. According to the invention, to prevent such an occurrence, during the switchover the controller output is first passed via the metering characteristic assigned to operation theretofore, and then via an inverted form of a metering characteristic assigned to the other type of operation. Jumps in the actuating signal curve during the switchover between full operation and partial shut-down are thereby avoided. The controller is initialized for the following type of operation and the actuating signal produced in the controller for the subsequent type of operation is set to the last-generated value of the actuating signal corresponding to the preceding controller, thereby ensuring a jump-free transition between the types of operation.
According to the invention, if the internal combustion engine is initially operated in full operation (ignition in all cylinders) and is switched to a partial shut-down mode (ignition is performed in only some of the cylinders) owing to a reduction in load, then, at the time of the switchover, the actuating signal produced in accordance with the full-operation metering characteristic is passed via the inverted partial shut-down operation metering characteristic, subjected to a further manipulation and finally passed to the actuator of the throttle valve.
If, on the other hand, the switchover is from partial shut-down to full operation, the actuating signal for partial shut-down operation is passed via the inverted full-operation metering characteristic and subjected to a subsequent manipulation before being passed on to the throttle-valve actuator. In both cases, the actuating signals correspond immediately before and after the switchover.
It is expedient if the manipulation of the actuator by means of the inverse metering characteristic is carried out just once at the time of the switchover. On completion of the switchover, the actuator is controlled by means of the controller assigned to the new type of operation, because it is no longer necessary to take into account the inverse metering characteristic.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.